Modern dental practice now applies precious and nonprecious metal castings in the mouth for restoration purposes by bonding these castings directly to the teeth. In some cases, such castings, covered with special plastic or porcelain coatings are installed to replace missing teeth, in which case they are bonded to other existing teeth. The bonding is carried out by applying epoxies or other glues that are designed to withstand the eroding action of saliva as well as provide the strength necessary for the teeth to function normally. However, in order for the epoxy to adhere to the castings, it is necessary to roughen the surface of the metal where it is to be bonded to provide a surface to which the epoxy may adhere. This process is generally accomplished by placing the casting into an etching solution where the etching process proceeds at an uneven rate over the surface to produce the desired rough surface.
Unfortunately, the etching solution also darkens the surface of the metal and in many cases leaves a residue which must be cleaned before applying the bonding material. Cleaning is accomplished by placing the metal in a separate cleaning solution for a sufficient period to remove the discoloration and residue, after which the metal is ready for bonding.
This process is shown in greater detail in FIG. 2. In this Figure, the etching apparatus is shown to comprise a first cleaning tank 201 having a cleaning solution 202, an agitator blade 209 supported by shaft 208 from motor 207, a plating electrode 206, a second plating electrode 203 having a head 205 which holds the casting piece 204. FIG. 2 also shows the cleaning apparatus to comprise a separate tank 210 containing a cleaning solution 215, a support rod 211 with a head 212 holding the dental casting piece 213 and an ultrasonic vibrator 214.
The etching process is usually carried out by placing the casting metal 204 in a solution of either sulfuric or nitric acid, passing a current through the electrode 203, the solution 202 and electrode 206 at a density which is typically 0.4 to 0.5 amperes per centimeter of the castings surface area. Current is passed in a direction which is opposite that used for plating. This process might be referred to as electrical deplating. The motor 207 operates the shaft 208 to drive the impeller 209 in order to keep a fresh supply of acid continually circulating about the surface of the casting metal. The time for this process is usually 2 to 5 minutes.
The cleaning operation is usually carried out as shown to the right in FIG. 2. The casting metal 213 is immersed in a cleaning solution 215 such as hydrochloric acid for a period of 10 minutes; however, if the casting metal is not sufficiently cleaned in this period, it is returned to the bath for another ten minute period. The head 212 of support rod 211 holds the casting metal 213 within the solution. The tank 210 holds the cleaning solution and also contains an ultrasonic vibrator 214 which operates to rapidly agitate the solution as well as vibrate the casting metal, accelerating the cleaning action.
Although there are a number of currently used cleaning and etching systems available, as evidenced by U.S. Pat. Nos. 2,616,820, 2,861,932 and 3,411,999, none appear applicable to provide the desired rough etching or speed the operations.
U.S. Pat. No. 2,616,820 is primarily concerned with cleaning using ultrasonic vibrations. U.S. Pat. No. 2,861,932 is directed at a method of treating semiconductors and primarily to etching the semiconductors using electric current in a vibrating bath at a reduced temperature. However, a cleaning operation is not carried out simultaneously with the etching operation in this invention, nor is there any indication of time reductions of the order necessary to provide an efficient dental operation.
U.S. Pat. No. 3,411,999 is directed to a method of etching metals uniformly along a surface. In this process, a current is passed through the metal to be etched. The metal is immersed in an etchant bath and the entire bath is vabrated. The vibration is intended to obtain a uniform surface rather than an uneven surface as is desired for a dental casting metal.
Generally, the prior art processes are designed to either reduce bulk or polish the metal, but none are directed at roughening the metal's surface with specific pitting depth, width and distribution density, all of which are required for satisfactory bonding with current available bonding agents.
The current method typically used by a dentist was so time consuming that it was impractical to etch and clean the metal and install the casting in one visit. This would result in an additional visit by the patient. The time needed to prepare a treatment room combined with the time for cleanup after a visit adds appreciable unproductive time to the procedure.
For example, the prior art total time for etching, bath transfer and cleaning amounted to 26 minutes, preventing installation of the casting in the usual one-half hour appointment period. The time lost in setup at the next visit is typically 9 minutes and includes cleanup of the room, greeting, and settling the patient in the chair. A process for producing the etched and cleaned metal in under 100 seconds is a serious need for the dental profession. The improvement in efficiency could be well over 1000 percent.